metadata: bad default value for IPv6 only environements

## Environment

IPv6 only kubernetes pods

## Code and Dependencies

Looking at this code:
https://github.com/googleapis/google-cloud-go/blob/18dec3a31f1d24dfa1dc58eeb532991336772dac/compute/metadata/metadata.go#L520-L527

Even in an IPv6 only environment, we will always try http://169.254.169.254. This means that users must set GCE_METADATA_HOST.

## Expected behavior

If IPv4 is not enabled but IPv6 is enabled, then we should default to the IPv6 address. Or we should try multiple values.

(I'm assuming we can tell from inside a kubernetes Pod that we are IPv6 only, in a reasonable efficient way)

## Actual behavior

We default to an IP address that we (could) know cannot work.

## Additional context

Setting GCE_METADATA_HOST to the correct value works (and Gemini suggests `export GCE_METADATA_HOST="fe80::1%eth0"`), but it feels like this is easy for us to fix in the SDK.

I am happy to send a PR if we agree that we can fix this, at least for the case when we _know_ that IPv4 is not enabled.